Crustal and Upper-Mantle Structure Beneath Ice-Covered Regions in Antarctica from S-Wave Receiver Functions and Implications for Heat Flow

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Crustal and Upper-mantle Structure Beneath Ice-covered Regions in Antarctica from S-wave Receiver Functions and Implications for Heat Flow

C. Ramirez The Pennsylvania State University

Andrew A. Nyblade The Pennsylvania State University

S. E. Hansen University of Alabama - Tuscaloosa

Douglas A. Wiens University of Alabama - Tuscaloosa

Sridhar Anandakrishnan The Pennsylvania State University

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Recommended Citation Ramirez, C. et al. (2015). Crustal and upper-mantle structure beneath ice-covered regions in Antarctica from S-wave receiver functions and implications for heat flow. Geophysical Journal International 204(3), 1636-1648. DOI: 10.1093/gji/ggv542

This Article is brought to you for free and open access by the College of the Sciences at ScholarWorks@CWU. It has been accepted for inclusion in Geological Sciences Faculty Scholarship by an authorized administrator of ScholarWorks@CWU. For more information, please contact [email protected]. Authors C. Ramirez, Andrew A. Nyblade, S. E. Hansen, Douglas A. Wiens, Sridhar Anandakrishnan, Richard C. Aster, Audrey D. Huerta, Partick Shore, and Terry Wilson

This article is available at ScholarWorks@CWU: https://digitalcommons.cwu.edu/geological_sciences/14 Geophysical Journal International

Geophys. J. Int. (2016) 204, 1636–1648 doi: 10.1093/gji/ggv542 GJI Seismology

Crustal and upper-mantle structure beneath ice-covered regions in Antarctica from S-wave receiver functions and implications for heat ﬂow

C. Ramirez,1 A. Nyblade,1 S.E. Hansen,2 D.A. Wiens,3 S. Anandakrishnan,1 R.C. Aster,4 A.D. Huerta,5 P. Shore3 and T. Wilson6 1Department of Geosciences, The Pennsylvania State University; University Park, PA 16802, USA. E-mail: [email protected] 2Department of Geosciences, The University of Alabama, Tuscaloosa, AL 35487,USA 3Department of Earth and Planetary Sciences, Washington University, St. Louis, MO 63130,USA 4Department of Geosciences, Colorado State University, Fort Collins, CO 80523,USA 5

Department of Geological Sciences, Central Washington University, Ellensburg, WA 98926,USA Downloaded from 6Byrd Polar Research Center and School of Earth Sciences, The Ohio State University, Columbus, OH 43210,USA

Accepted 2015 December 16. Received 2015 December 7; in original form 2015 August 18 http://gji.oxfordjournals.org/ SUMMARY S-wave receiver functions (SRFs) are used to investigate crustal and upper-mantle structure beneath several ice-covered areas of Antarctica. Moho S-to-P (Sp) arrivals are observed at ∼6–8 s in SRF stacks for stations in the Gamburtsev Mountains (GAM) and Vostok Highlands (VHIG), ∼5–6 s for stations in the Transantarctic Mountains (TAM) and the Wilkes Basin (WILK), and ∼3–4 s for stations in the West Antarctic Rift System (WARS) and the Marie Byrd Land Dome (MBLD). A grid search is used to model the Moho Sp conversion time with Rayleigh wave phase velocities from 18 to 30 s period to estimate crustal thickness and at The University of Montana on September 19, 2016 mean crustal shear wave velocity. The Moho depths obtained are between 43 and 58 km for GAM, 36 and 47 km for VHIG, 39 and 46 km for WILK, 39 and 45 km for TAM, 19 and 29 km for WARS and 20 and 35 km for MBLD. SRF stacks for GAM, VHIG, WILK and TAM show little evidence of Sp arrivals coming from upper-mantle depths. SRF stacks for WARS and MBLD show Sp energy arriving from upper-mantle depths but arrival amplitudes do not rise above bootstrapped uncertainty bounds. The age and thickness of the crust is used as a heat flow proxy through comparison with other similar terrains where heat flow has been measured. Crustal structure in GAM, VHIG and WILK is similar to Precambrian terrains in other continents where heat flow ranges from ∼41 to 58 mW m−2, suggesting that heat flow across those areas of East Antarctica is not elevated. For the WARS, we use the Cretaceous Newfoundland–Iberia rifted margins and the Mesozoic-Tertiary North Sea rift as tectonic analogues. The low-to-moderate heat flow reported for the Newfoundland–Iberia margins (40–65 mW m−2) and North Sea rift (60–85 mW m−2) suggest that heat flow across the WARS also may not be elevated. However, the possibility of high heat flow associated with localized Cenozoic extension or Cenozoic-recent magmatic activity in some parts of the WARS cannot be ruled out. Key words: Heat flow; Seismicity and tectonics; Site effects; Antarctica.

et al. 2012). In the absence of direct borehole measurements, one 1 INTRODUCTION approach to estimating heat flow is to use crustal and upper-mantle The stability of the West Antarctic Ice Sheet (WAIS) and other structure as a proxy via comparison to tectonically similar regions large glaciers in Antarctica, although of great importance because around the world where heat flow has been measured. Indeed, of their potential to raise sea level by several meters, is poorly the use of heat flow from terrains with similar structure and age understood, in part because there are very few heat flow measure- as a proxy is commonplace (e.g. Pollack et al. 1993;Artemieva ments available for constraining models of ice sheet behaviour (e.g. & Mooney 2001; Pollard et al. 2005; Davies & Davies 2010; Parizek 2002; Bennett 2003; Pollard et al. 2005; Pattyn 2010; Larour Davies 2013).

1636 C The Authors 2016. Published by Oxford University Press on behalf of The Royal Astronomical Society. Crust and upper-mantle structure of Antarctica 1637

Obtaining estimates of crustal and upper-mantle structure in The initiation of rifting in the WARS is not well known but prob- Antarctica needed for applying this approach, however, can be chal- ably started in the Jurassic when Africa separated from EA, with lenging. Harsh environmental conditions make it difficult to obtain a primary phase of extension circa 105–85 Ma during the breakup seismic data, especially in the interior of the continent, and where and opening of the Southern Ocean between WA and New Zealand seismic data have been collected, reverberations within the thick (Behrendt et al. 1991; Tessensohn & Woorner¨ 1991; Siddoway ice sheets often make the use of standard data analysis methods for 2008). Extension during the Cretaceous rifting phase was >100 imaging crustal and upper-mantle structure difficult. In particular, per cent, resulting from pure shear deformation orthogonal to the reverberations within the ice layer can mask key converted seis- TAM (Siddoway 2008 and references therein). Renewed extension mic phases from deeper discontinuities, such as the Moho or the in the Cenozoic within the western WARS has been documented lithosphere–asthenosphere boundary (LAB). from sea floor spreading in the Adare trough, reconstructions of In order to improve our understanding of crustal and upper-mantle plate circuits, and the development of the Terror rift within the Vic- structure in Antarctica, and to use this understanding for inferring toria Land Basin (Cande & Stock 2004;Muller¨ et al. 2007; Stuart heat flow, S-wave receiver functions (SRFs) are modelled together et al. 2007; Fielding et al. 2008; Wilson & Luyendyk 2009; Granot with Rayleigh wave phase velocity measurements to image crustal et al. 2010), with possibly up to 150 km of extension between 68–46 and upper-mantle structure beneath many broad-band seismic sta- Ma (Cande & Stock 2004; Wilson & Luyendyk 2009). However, tions deployed on ice. Imaging earth structure beneath glaciers using how much of the Cenozoic extension affected the Central and East- SRFs can be less complicated than P-wave receiver function (PRF) ern Basins of the western WARS, as well as the eastern WARS, Downloaded from imaging (e.g. Chaput et al. 2014) because SRFs do not contain re- remains uncertain (e.g. Karner et al. 2005), with some studies even verberations from the ice layer (Hansen et al. 2009a, 2010). SRFs predicting compression within the eastern WARS during the Eocene can also often provide information about discontinuities in the up- and Oligocene (Granot et al. 2010). per mantle not apparent in PRFs (e.g. Rychert et al. 2005; Hansen Several of the deep basins in the eastern WARS (e.g. Byrd Sub- et al. 2009b;Fordet al. 2010). glacial Basin, the Bentley Subglacial Trench and the Pine Island

Here we apply the SRF method systematically to data from trough), which have locally much thinner crust than surrounding ar- http://gji.oxfordjournals.org/ seismic stations deployed on ice in Antarctica, incorporating boot- eas of the eastern WARS (e.g. Winberry & Anandakrishnan 2004; strapped error bounds on the SRF stacks to obtain uncertainty es- Chaput et al. 2014), have been interpreted as Cenozoic rift basins timates on models of crustal and upper-mantle structure. Results (e.g. LeMasurier 2008; Jordan et al. 2010). A seismic low wave obtained for areas in East Antarctica (EA) and West Antarctica speed anomaly in the upper mantle beneath the Bentley Trench (WA) are then compared to similar terrains in other parts of the supports that interpretation (Lloyd et al. 2015). However, Wanna- globe where heat ﬂow has been measured to comment on heat ﬂow maker et al. (1996) reported high crustal and upper-mantle resis- in Antarctica. tivity values beneath the Byrd basin, suggestive of an inactive rift,

and similar deep basins have been imaged beneath the Central and at The University of Montana on September 19, 2016 Eastern Basins of the western WARS which show little evidence for 2 TECTONIC BACKGROUND Cenozoic extension (Trey et al. 1999; Karner et al. 2005). There is also little evidence for present-day extension within the WARS. Antarctica consists of three first-order tectonic domains: WA, EA Overall seismicity is low to non-existent (Winberry & Anandakr- and the Transantarctic Mountains (TAM), which separate EA from ishnan 2003; Lough et al. 2013), and GPS data resolve little, if any, WA. In this section, the relevant geology and physiography of these horizontal strain (Donnellan & Luyendyk 2004; Wilson et al. 2014). regions are briefly reviewed. MBLD consists mainly of a low-relief dome capped by several Cenozoic volcanoes (Mukasa & Dalziel 2000). Although the most common rock type exposed is alkaline basalt, the basalts are covered 2.1 West Antarctica in many places by felsic lavas forming N–S and E–W oriented Although WA is composed of several tectonic blocks (Dalziel & volcanic chains (LeMasurier & Rex 1989). A suite of mafic dykes Elliot 1982), the data used in this study come from just two of them: across the MBLD was used by Storey et al. (1999) to infer that a mid- the West Antarctic Rift System (WARS) and the Marie Byrd Land Cretaceous mantle plume could still be situated underneath MBLD Dome (MBLD; Fig. 1). The WARS is one of the most extensive today. Recent body and surface wave tomography images show physiographic features in WA (Fig. 1), extending from the Ross an upper-mantle low-velocity structure beneath MBLD, consistent Sea to the Antarctic Peninsula, covering a distance of ∼3000 km with a mantle plume interpretation (Hansen et al. 2014;Lloydet al. (Behrendt et al. 1991). It is bound to the north and south by the 2015; Heeszel et al. 2016), and mantle transition zone thinning MBLD and the TAM, respectively. The size and tectonic setting imaged beneath the Ruppert Coast of Marie Byrd Land suggests a of the WARS have been often compared to that of the Cenozoic possible lower-mantle plume source (Emry et al. 2015). Basin and Range province in the continental U.S., the Cenozoic East African Rift System, the Mesozoic-Tertiary North Sea Rift, and the Jurassic-Cretaceous Newfoundland–Iberia passive margins 2.2 East Antarctica (Behrendt et al. 1991; Tessensohn & Woorner¨ 1991; Behrendt 1999; LeMasurier 2008). The WARS section between the MBLD and the Because a thick ice sheet covers most of EA, its geologic compo- TAM is referred to as the eastern WARS, and the section in the sition and tectonic history remain enigmatic (Adie 1962; Dalziel Ross Sea as the western WARS. The western WARS consists of 1992). Most of EA is thought to be a large and stable Precambrian three basins, the Victoria Land Basin, immediately adjacent to the craton, comprised of Proterozoic and Archean terrains (Moores TAM, and the Central and Eastern Basins outboard to the north. 1991; Dalziel 1992; Fitzsimons 2000). The physiographic regions The eastern WARS is characterized topographically by several deep of EA relevant to this study are the Gamburtsev Mountains (GAM), basins, most prominently the Byrd Subglacial Basin and the Bentley the Vostok Highlands (VHIG), and the Wilkes Basin (WILK) Subglacial Trench (Fig. 1). (Fig. 1). 1638 C. Ramirez et al. Downloaded from http://gji.oxfordjournals.org/ at The University of Montana on September 19, 2016

Figure 1. Location of seismic stations used in this study and bedrock elevations from BEDMAP2 (Fretwell et al. 2013). The coloured lines show the grouping of stations explained in the text. Upper Gamburtsev Mountains (GAMU), Middle Gamburtsev Mountains (GAMM), Vostok Highlands (VHIG), Transantarctic Mountains (TAM), West Antarctic Rift System (WARS) and Marie Byrd Land Dome (MBLD). Thin black lines show the location of the cross-sections A–A, B–B,C–C and D–D in Fig. 5.

The GAM, situated near the centre of EA, have high and relatively overlain by the Beacon supergroup (Fitzgerald 1992). Lacking any fresh alpine topography (Ferraccioli et al. 2011) and are supported evidence for a compressional origin, the TAM are the largest non- by a thick crustal root (Hansen et al. 2010). We separate the GAM collisional mountain range in the world, and their origin remains into upper (GAMU) and middle (GAMM) sections based on sub- widely debated (e.g. Fitzgerald et al. 1986; Stern & ten Brink 1989; glacial topography to facilitate discussion of the results (Fig. 1). ten Brink et al. 1997; Studinger et al. 2004; Huerta & Harry 2007). The WILK is a 1250 km long, 100–400 km wide subglacial feature (Drewry 1976) located adjacent and subparallel to the TAM. The VHIG encompass the region between the GAM and the WILK. 3DATA This study utilizes data from three different experiments that include numerous broad-band seismic stations that have been deployed on 2.3 Transantarctic Mountains ice across Antarctica (Fig. 1). The Transantarctic Mountains Seis- The TAM are ∼200 km wide, ∼4kmhighandextend∼4000 km mic Experiment (TAMSEIS) consisted of 42 seismic stations de- across the interior of Antarctica, forming a boundary between EA ployed between 2000 and 2003 in three arrays: (1) a 16 station east– and WA.Uplift of the mountains, constrained by apatite fission track west oriented array with ∼20 km interstation spacing, including analysis, began in the early Cenozoic (Fitzgerald 1992). Basement 10 ice stations (E012-E030), (2) a 17 station north–south oriented rocks in the TAM consist mainly of Precambrian and Cambrian array with ∼80 km interstation spacing, with all stations deployed meta-sediments and Cambro-Ordovician granites, unconformably on ice (N000-N132) and (3) a nine station array deployed on rock Crust and upper-mantle structure of Antarctica 1639 spread along the coastal area near Ross Island (Lawrence et al. 2006; of the ice layer was taken from BEDMAP2 (Fretwell et al. 2013), Watson et al. 2006). The Gamburtsev Mountains Seismic Experi- and the S-wave velocity of the ice was fixed at 1.9 km s−1 (Kohnen ment (GAMSEIS) consisted of 26 stations deployed on ice (Lloyd 1974). Crustal thickness was varied from 10 to 70 km, with a 1 km et al. 2013). Twelve stations were installed to extend the north–south increment, and the average crustal shear wave velocity was varied TAMSEIS array, with two reoccupied stations (N124 and N132). from 3.4 to 3.9 km s−1, with a 0.1 km s−1 increment. The models A subparallel array was also deployed south of the north–south were constrained so that velocities in the upper crustal layer had transect, which included seven stations, and another six stations to be equal to or less than the lower layer. Given that mantle ve- were installed that formed a crossing transect to improve 2D data locities in EA are higher than in WA (e.g. Danesi & Morelli 2001; coverage (Fig. 1). We also use data from POLENET-ANET (WA Ritzwoller et al. 2001;Lloydet al. 2013;Lloydet al. 2015), the and TAM portions of the Polar Earth Observing Network; Fig. 1). mantle was modelled as a half-space with a fixed S-wave velocity This deployment consisted of 16 rock stations and 19 ice stations, of 4.5 km s−1 for EA and the TAM, and 4.3 km s−1 for WA. The including a backbone array that was deployed between 2008 and density for each layer was calculated using an empirical S-wave 2012 and a temporary transect that was deployed between 2010 and velocity versus density relationship from Brocher (2005), and the 2012 (Chaput et al. 2014). All POLENET-ANET ice station data P-wave velocity was calculated using the S-wave velocity and a cor- that became available after the 2013–2014 field season have been responding Poisson’s ratio (0.33 for the ice layer, 0.25 for the crust used in this study. For all three experiments (TAMSEIS, GAM- and 0.28 for the mantle). A summary of these values can be found in SEIS and POLENET), the stations were equipped with broad-band Table 1. Downloaded from seismometers, and data were recorded at 40 samples per second. Each model was used to compute Rayleigh wave phase velocities for periods between 18 and 30 s, and the modelled dispersion curves were then compared to Rayleigh wave phase velocities from Heeszel 4 METHODOLOGY et al. (2016), which were smoothed using a three-point moving average. However, even after applying the three-point moving average,

4.1 Receiver function computation some dispersion curves still exhibited unrealistic changes in phase http://gji.oxfordjournals.org/ velocity over a small period range. Within each tectonic region SRFs were generated using the method outlined by Hansen et al. (Fig. 1), the individual dispersion curves were similar, and there- 2009a, 2010), with broad-band data from earthquakes with magni- fore to further smooth the phase velocity curves used in the grid tudes >5.5 M , distances between 60◦ and 80◦, and focal depths w search, we computed an average dispersion curve for each region <200 km. This limited range of events eliminates many of the (Fig. 1). The maximum deviation between phase velocities in the phases that could interfere with the Sp Moho converted phase, as unsmoothed, individual dispersion curves for each region and the described by Wilson et al. (2006). The seismograms were visually corresponding regional phase velocity average was ≤0.08 km s−1. inspected to select events with relatively high signal-to-noise ratios ± −1 Thus, for the grid search, we used an error bound of 0.08 km s at The University of Montana on September 19, 2016 and clear S-wave arrivals. The traces were rotated from the north– for the phase velocity at each period, with the phase velocity taken east-vertical coordinate system to the radial–transverse-vertical co- from the average dispersion curve for the region. ordinate system and were cut into segments extending from 100 s Models that yielded synthetic dispersion data within error of the prior to the S-phase arrival to 12 s after. The receiver functions were observed phase velocities were then used in the second step of the computed by deconvolving the radial component from the vertical grid search, in which predicted Moho Sp times for the models were using the iterative time domain method described by Ligorr´ıa & Am- compared to the observed times in the SRF stacks. The correspond- mon (1999), and the time and amplitude of the SRFs were reversed ing uncertainty used to fit the conversion timing is provided by the to make them visually comparable to PRFs. Receiver functions with bootstrapped error bounds on the SRF stacks. Only models that fit high noise levels were removed from the data set, and the remaining both the observed dispersion data and the observed Moho Sp time receiver functions at each station were stacked. We used a stacking within error were considered to be successful. Examples for two procedure that utilizes the bootstrapping method described in Efron stations, one with thick crust (P124) and the other with thin crust & Tibshirani (1991) to compute error bounds. Receiver functions (ST03), are shown in Fig. 2. in each stack were randomly resampled 200 times to obtain 95 per cent error bounds, which are shown on the SRF stacks (Fig. 2). New data were available for five GAMSEIS stations (GM02, GM05, N140, N215 and P061) compared to the previous analysis 5 RESULTS of these stations by Hansen et al. (2010). For these stations, we used the same events as Hansen et al. (2010) but added between 4 5.1 Crustal structure and 37 new events per station. For the GAMSEIS and TAMSEIS Crustal thickness estimates across Antarctica, which range from 19 stations where no new data were available, we used the same events to 59 km, are summarized in Fig. 3 and Table 2, and individual sta- as Hansen et al. (2009a, 2010) and recomputed the stacks to obtain tion results are provided in the Supporting Information. Histograms bootstrap error bounds. SRFs have not been previously reported for are used to show the distribution of the models that fit both the dis- the POLENET stations. persion curves and the Moho Sp arrival time within the prescribed error, and the standard deviation of these models provides an estimate of crustal parameter uncertainty for each station (Fig. 2). 4.2 Grid search for crustal structure However, this estimation does not include uncertainties that arise Also following the procedure described by Hansen et al. (2009a, from the fixed ice thickness, ice shear-wave velocity, or crustal den- 2010), a two-step grid search approach was used to estimate the sity used in the grid search. As described by Hansen et al. (2009a), crustal thickness and average crustal shear wave velocity beneath uncertainties associated with these parameters factor into the over- each station. In the first step, we generated 4-layer models to repre- all uncertainty of the crustal structure estimates, averaging 1.5 km sent the ice, the upper and lower crust and the mantle. The thickness for crustal thickness and 0.04 km s−1 for average crustal shear wave 1640 C. Ramirez et al. Downloaded from http://gji.oxfordjournals.org/ at The University of Montana on September 19, 2016

Figure 2. Data and modelling results for two stations, ST03 and P124. Panels ‘A’ show the phase velocity dispersion curves for all models (red lines) that ﬁt the dispersion data provided by Heeszel et al. (2016; blue lines with error bars). Panels B and C show the distribution of the average crustal shear velocities and Moho depths for models that ﬁt the dispersion data and Moho Sp timing in the SRF stacks. N denotes the total number of models. Panels D show the SRF stack, where the black line is the mean of the stack and the grey shaded areas show the bootstrap error bounds. N denotes the number of individual SRFs contributing to each stack.

Table 1. Parameters used in the grid search. Thickness (km) Vp (km s−1)Vs(kms−1) Density (g cm−3) Poisson’s ratio (v)

Ice BedMap2 1.9 0.33 2(v−1) × + Upper crust 10–70 Vs 2v−1 3.4–3.9 0.32 Vs 0.77 0.25 Lower crust 10–70 3.4–3.9 0.25 Mantle Half-space 4.3 or 4.5 0.28 velocity. By combining these estimates with the standard deviation GAMU, is located just grid north of the GAM, where the average of the accepted models, the errors in crustal thickness can, at most crustal thickness is 49 km and the average crustal S-wave velocity stations, be estimated to within 3 km and the mean crustal veloc- is 3.8 km s−1. In the central portion of the GAM (GAMM), the ity to within 0.1 km s−1. However, for some stations, the standard average crustal thickness is 54 km, with an average velocity of 3.8 deviation of the accepted models is greater than 3 km (Fig. 3 and km s−1. Moving grid south, the VHIG region has an average crustal Supporting Information). We chose the larger of the two numbers thickness of 44 km and velocity of 3.7 km s−1, while the WILK in every case as our ﬁnal estimated error for each station (Table 2). has an average crustal thickness of 45 km and a velocity of 3.8 To facilitate a comparison of results, we grouped stations by km s−1. In the TAM, at roughly −77◦S, stations in the east–west physiographic region, as shown in Figs 1 and 3. The ﬁrst region, TAMSEIS transect are underlain by crust with an average thickness Crust and upper-mantle structure of Antarctica 1641 Downloaded from http://gji.oxfordjournals.org/

Figure 3. Summary of results. Green squares show the crustal thickness results for stations in West Antarctica and red squares are the results for East Antarctica. Grey squares represent previous studies from Hansen et al. 2009a, 2010) in East Antarctica and from Chaput et al. (2014) in West Antarctica. The brown and blue triangles denote bedrock and ice elevation, respectively. Stations in East Antarctica highlighted with black rectangles are stations where additional seismic data was used for computing SRF stacks compared to Hansen et al. (2010). Acronyms of the regions are explained in Fig. 1. at The University of Montana on September 19, 2016 of 43 km and a mean shear velocity of 3.8 km s−1. The WARS region 6 DISCUSSION displays much thinner crust, averaging 23 km, with an average S- −1 In summary, Sp arrivals from the Moho are evident on the SRF wave velocity of 3.6 km s . Grid south of the WARS, the MBLD stacks for most stations, and by combining Rayleigh wave phase displays average crustal thickness of 25 km and an average shear −1 velocity measurements with the arrival times of the Moho Sp phase, wave velocity of 3.6 km s . we have been able to estimate Moho depths and mean crustal shear velocities. In contrast, the SRF stacks provide little interpretable information about upper-mantle structure. We therefore focus the discussion on a comparison of our results to those from previous 5.2 Upper-mantle structure studies of crustal structure, and then use our results to comment on Inspection of the SRF stacks shows that roughly half of the stations heat flow in EA and WA. display a trough, labelled in Fig. 2, within the time window where an Sp conversion from an upper-mantle discontinuity, such as the LAB, would be expected (i.e. ∼1–10 s after the Moho Sp arrival). 6.1 Crustal structure in West Antarctica However, only a few individual stations display such an Sp signal with an amplitude rising above the bootstrapped error bounds, and Profile B–B in Fig. 5 shows a cross-section across WA, extending so the individual station stacks cannot be confidently interpreted roughly from Mt. Whitmore to MBLD. The deepest Moho depths (see Supporting Information for individual stations). are observed under Mt. Whitmore and Mt. Sidley (PRF estimates In an attempt to improve the associated signal-to-noise ratio, we from Chaput et al. 2014), while the shallowest Moho is observed stacked the individual receiver functions within each physiographic under stations WAIS and ST04 (19 km depth) in the Bentley Sub- region (Fig. 1). All regional stacks show an Sp arrival from the Moho glacial Trench. The crustal thicknesses for stations away from the (Fig. 4). However, the regional stacks from EA and the TAM show Bentley Trench range from 20 to 26 km. These results are similar to little to no indication of an Sp arrival from the LAB, indicating that previously published PRF results from Chaput et al. (2014), except the LAB may be a gradational boundary beneath these areas. The for three stations where the results do not fall within the reported stacks for the WARS and MBLD stations in WA have a relatively uncertainties (Fig. 3). Our results for stations ST09 and ST14 differ long wavelength (∼10 s), low amplitude negative arrival between by 6 km from Chaput et al. (2014), and for station ST02, the dif- ∼5 and 15 s, suggesting that some Sp energy may be arriving ference is 9 km. SRF stacks for these stations show a clear Moho from the LAB; however, similar to most of the stacks for individual Sp arrival, and the associated error bounds are comparable to those stations, the signal amplitude does not rise above the bootstrapped for other stations (see the Supporting Information). Therefore, it is error bounds and thus we also do not have confidence in interpreting not apparent why there is a notable difference in our Moho depth the WARS and MBLD stacks. estimates for these stations compared to Chaput et al. (2014). 1642 C. Ramirez et al.

Table 2. Summary of results obtained in this study. Station Latitude Longitude Crustal thickness (km) Vs (km s−1) Elevation (km) Ice thickness (km) E012 −77.05 159.32 41 ± 4 3.7 1.92 0.24 E014 −76.99 158.62 43 ± 8 3.7 2.09 0.69 E018 −76.82 157.22 40 ± 3 3.7 2.09 1.90 E020 −76.73 156.54 44 ± 3 3.8 2.12 1.63 E024 −76.54 155.23 45 ± 3 3.8 2.21 1.51 E028 −76.31 154.04 45 ± 4 3.8 2.25 1.65 E030 −76.25 153.38 45 ± 3 3.8 2.26 1.79 JNCT −76.93 157.90 42 ± 3 3.8 2.13 1.48 N020 −77.47 155.89 40 ± 5 3.7 2.28 1.52 N036 −78.55 151.29 42 ± 3 3.7 2.24 2.58 N044 −79.07 148.62 44 ± 3 3.7 2.27 2.34 N060 −80.00 142.60 46 ± 3 3.8 2.26 2.80 N076 −80.81 135.43 47 ± 3 3.8 2.48 2.59 N100 −81.65 122.61 44 ± 5 3.7 2.93 2.56 N108 −81.88 117.61 45 ± 3 3.8 3.06 2.66

N116 −82.01 112.57 44 ± 5 3.7 3.18 2.33 Downloaded from N124 −82.07 107.64 45 ± 5 3.8 3.33 2.50 N132 −82.06 101.96 44 ± 5 3.8 3.42 3.55 N140 −82.01 96.77 46 ± 4 3.8 3.57 3.03 N156 −81.67 86.50 48 ± 5 3.7 3.84 2.84 N165 −81.41 81.76 54 ± 5 3.8 3.97 2.47

N173 −81.11 77.47 56 ± 5 3.8 4.07 2.74 http://gji.oxfordjournals.org/ N182 −80.74 73.19 56 ± 4 3.8 4.05 2.64 N190 −80.33 69.43 48 ± 5 3.8 3.92 3.09 N198 −79.86 65.96 49 ± 5 3.8 3.78 3.06 N206 −79.40 62.86 48 ± 4 3.8 3.66 2.96 N215 −78.91 59.99 49 ± 6 3.8 3.55 3.07 GM01 −83.99 104.73 35 ± 3 3.6 3.27 3.30 GM02 −79.43 97.58 42 ± 5 3.7 3.72 2.74 GM03 −80.22 85.94 54 ± 5 3.8 3.92 2.33 GM04 −83.00 61.11 49 ± 4 3.8 3.77 3.00 at The University of Montana on September 19, 2016 GM05 −81.18 51.16 49 ± 6 3.8 3.77 3.53 P061 −84.50 77.22 43 ± 5 3.7 2.51 2.96 P071 −83.65 77.33 42 ± 4 3.7 3.65 2.18 P080 −82.81 77.36 49 ± 5 3.7 3.81 1.77 P090 −81.94 77.31 59 ± 3 3.8 4.01 2.25 P116 −79.57 77.06 55 ± 4 3.8 3.93 1.75 P124 −78.87 77.66 57 ± 4 3.8 3.61 1.65 ST01 −83.23 −98.74 24 ± 3 3.5 2.03 2.24 ST02 −82.07 −109.13 23 ± 3 3.6 1.79 2.14 ST03 −81.41 −113.15 20 ± 3 3.5 1.66 2.23 ST04 −80.72 −116.58 19 ± 3 3.5 1.52 2.96 ST06 −79.33 −121.82 26 ± 3 3.7 1.52 2.40 ST07 −78.64 −123.79 26 ± 3 3.7 1.59 2.61 ST08 −77.95 −125.53 25 ± 3 3.6 1.78 2.30 ST09 −76.53 −128.47 24 ± 3 3.6 2.25 2.14 ST10 −75.81 −129.75 25 ± 3 3.6 1.75 1.13 ST12 −76.90 −123.82 20 ± 3 3.5 2.20 1.92 ST13 −77.56 −130.51 24 ± 3 3.6 1.86 1.98 ST14 −77.84 −134.08 34 ± 4 3.7 1.64 1.73 UPTW −77.58 −109.04 29 ± 4 3.7 1.33 2.39 DNTW −76.46 −107.78 20 ± 3 3.5 1.03 2.11 WAIS −79.42 −111.78 19 ± 3 3.5 1.80 3.37

Other previous seismic studies of crustal thickness in the WARS which the crustal thinning is observed, on the order of 100 km, as have been more localized or else have focused on the Ross Sea well as the total amount of crustal thinning (∼10 km) is similar region in WA. Winberry & Anandakrishnan (2004) found crustal to what we find for the Bentley Trench. The crustal shear velocity thicknesses of 21–31 km beneath the WARS, with the thinnest crust model from Trey et al. (1999) shows velocities ranging from 3.0 to (21 km) under the Bentley Trench, comparable to our findings. 3.6 km s−1 in the upper crust and 3.6 to 4.1 km s−1 in the lower crust, Trey et al. (1999) reported crustal thinning from ∼24 to ∼14 km comparable to the average crustal shear velocities ranging from 3.5 under the Central Trough and the Eastern Basin in the western to 3.7 km s−1 for stations ST02–ST08. The similarities in both WARS. While the spatial resolution of the Trey et al. (1999) study crustal thickness and shear velocity structure between the Ross Sea is better than in our cross-section (Fig. 5), the horizontal scale over sector of the WARSand eastern WARS suggest that first-order basin Crust and upper-mantle structure of Antarctica 1643 Downloaded from http://gji.oxfordjournals.org/ at The University of Montana on September 19, 2016

Figure 4. Regional SRF stacks. The black lines denote the mean of the stacks and the grey shaded areas are the bootstrap error bounds. The Moho Ps phase is clearly observed in all panels. WARS and MBLD SRF stacks show energy arriving in the 5–15 s window that may be coming from upper-mantle discontinuities. structures could be related and/or were created by similar processes. 6.2 Crustal structure in East Antarctica Our ﬁndings are also broadly consistent with gravity-derived crustal Proﬁles A–A and C–C in Fig. 5 show thick crust beneath the thickness estimates from O’Donnell & Nyblade (2014), differing by GAMU and GAMM regions. Stations N215–N190 are underlain by an average of only ∼5 km. These differences are most prominent 47–49 km thick crust that increases to 54–56 km beneath stations within MBLD and the Bentley Trench. 1644 C. Ramirez et al. Downloaded from http://gji.oxfordjournals.org/ at The University of Montana on September 19, 2016

Figure 5. Cross-section proﬁles showing ice surface elevations, bedrock elevations and Moho depths. Labels are the same as in Fig. 3. The locations of the cross-sections are shown in Fig. 1. Grey symbols give Moho depths at rock stations determined from P-wave receiver functions (Finotello et al. 2011; Chaput et al. 2014). Open circles are shown on the proﬁles for stations where no SRF Moho depth determinations were obtained (same stations as indicated in Fig. 3).

N182–N165. The thickest crust is also coincident with the highest Bennett 2003; Pollard et al. 2005; Pollard & DeConto 2009; Pattyn bedrock elevations in the GAM. The VHIG are underlain by uni- 2010), and the WAIS in particular may be susceptible to collapse in formly thick crust (45–47 km), but the crust under the WILK thins part because of possibly elevated heat flow in WA(Blankenship et al. from 46 km beneath station N076 to 40 km beneath station N020. 1993; Schroeder et al. 2014; Fisher et al. 2015). There are only a few Stations across the TAM (Fig. 5,profileD–D) display crustal thick- measurements of heat flow from the continent. Clow et al. (2012) nesses of 40–42 km, while stations grid east of station E018 display reported a single measurement of 240 mW m−2 from a deep ice somewhat thicker crust (44–45 km). core site in the middle of the WAIS. Borehole measurements from All of the crustal thickness estimates from the GAMU, GAMM, Siple Dome yielded a heat flow of 69 mW m−2 (Engelhardt 2004), VHIG, WILK and TAM regions match previous results from Morin et al. (2010) reported a heat flow of 115 mW m−2 from a Hansen et al. (2009a, 2010) within reported uncertainty estimates borehole located on the McMurdo Ice Shelf, and Fisher et al. (2015) (Fig. 3). Differences of a few kilometres in crustal thickness recently reported a heat flow of 285 mW m−2 below subglacial Lake for some stations can be attributed to the different error bounds Whillhans. used for the Moho Sp arrival time, differences in Rayleigh wave Proxy methods have also been used to estimate heat flow in phase velocities and, in some cases, the addition of new data, Antarctica. From modelling magnetic data, Fox-Maule et al. (2005) which yielded slightly different Moho Sp times. The results pre- suggested that heat flow in EA is ∼50 to 60 mW m−2 and ∼55 sented here are also in agreement with gravity-derived crustal to 65 mW m−2 in WA, with slightly higher heat flow near areas thickness estimates from O’Donnell & Nyblade (2014). On av- with Cenozoic-recent volcanoes and the Siple coast. Pollard et al. erage, our results in EA differ from their estimates by less than (2005) estimated heat flow using global average values for similar 4 km. aged terrains as those found in Antarctica. In their heat flow map, estimatesrangedfrom40to55mWm−2 inEAand60to70mWm−2 in WA. Blankenship et al. (1993) reported elevated heat flow in WA from interpreting aerogeophysical data, including extremely high 6.3 Implications for heat flow heat flow of 10–25 W m−2, which they attributed to an isolated Geothermal heat flux is an important factor that can greatly influence volcanic vent. And in another study of WA, Schroeder et al. (2014) the stability of ice sheets (e.g. Huybrechts et al. 1993; Parizek 2002; used airborne radar to infer a high heat flow of ∼114 ± 10 mW m−2 Crust and upper-mantle structure of Antarctica 1645 for the Thwaites Glacier hydrologic catchment, with heat flow in Thus, using the Newfoundland–Iberia passive margins and the a few areas exceeding 200 mW m−2. They attributed the elevated North Sea rift as tectonic analogues to the WARS, it can be argued heat flow to magmatic activity. that heat flow in the WARS need not be elevated, consistent with To examine the implications of our findings for heat flow across the findings of Fox-Maule et al. (2005), who reported heat flow for Antarctica, we compare our crustal thickness estimates in three the WARS of ∼55–65 mW m−2. This conclusion, however, does areas of EA and one in WA to estimates from other tectonically not preclude the possibility of elevated heat flow localized within similar terrains where heat flow has been measured. A number of narrow Cenozoic rifts that may have developed subsequently to authors (e.g. Tessensohn & Woorner¨ 1991; LeMasurier 2008)have the broader Cretaceous rift system, or in areas of Cenozoic-recent suggested as possible analogues to the WARS the East African rift magmatic activity within the WARS. system and the Basin and Range province, which have eleveated In EA, most of the crust is either Proterozoic or Archean in age, heat flow (Morgan 1982), and the Newfoundland–Iberia passive as reviewed previously. Precambrian crust typically exhibits Moho margins and the North Sea rift, which do not have elevated heat depths of ∼35 to 45 km (Christensen & Mooney 1995; Rudnick flow. The Cenozoic Basin and Range province in North America is & Fountain 1995; Zandt & Ammon 1995; Tugume et al. 2013; associated with thicker crust than the WARS, ranging from ∼28 to Kachingwe et al. 2015). Crustal thicknesses in EA, outside of the ∼35 km (Zandt et al. 1995). The Rio Grande rift in central New GAM, average ∼40 to 45 km, which is consistent with an inter- Mexico, on the eastern edge of the southern Basin and Range, also pretation of Proterozoic and/or Archean crust beneath the EA ice has thicker crust than the WARS, with an average Moho depth of sheet. Average global heat flow estimates in Proterozoic terrains Downloaded from ∼35 km (Wilson et al. 2005). Within the Cenozoic East African rift vary between 46 to 58 mW m−2 and in Archean terranes between system, the Kenya rift has Moho depths ranging from ∼25 to 44 km 41 to 51 mW m−2 (Nyblade & Pollack 1993; Pollack et al. 1993). with an average of ∼35 km (Prodehl et al. 1997 and references Therefore, heat flow across much of EA is likely in the 41–58 mW therein), and in the Main Ethiopian Rift, Moho depths range from m−2 range. This conclusion is consistent with heat flow estimates 26 to 40 km (Maguire et al. 2006). Other well-studied Cenozoic from Pollack et al. (1993), Fox-Maule et al. (2005), Davies (2013), rifts with elevated heat flow, such as the Baikal rift and the Rhine and Pollard et al. (2005). http://gji.oxfordjournals.org/ Graben (Morgan 1982), also have thicker crust than the WARS (e.g. 38–42 km for the Baikal rift and 30 km for the Rhine Graben; Brun et al. 1992; ten Brink & Taylor 2002). Because there are significant 7 SUMMARY differences in crustal thicknesses between these Cenozoic rifts and Crustal thickness and shear velocity structure across Antarctica the WARS, they are probably associated with different extensional have been investigated by modelling SRFs and surface wave mea- processes and histories and therefore may not be good analogues surements for temporary stations deployed on ice. In EA, we find for the WARS. 43–58 km thick crust beneath the GAM, 39–46 km thick crust be-

The Newfoundland–Iberia passive margins formed mainly in at The University of Montana on September 19, 2016 neath the WILK, and 36–47 km thick crust beneath the VHIG. In the the Cretaceous, similar to the WARS. Crustal thicknesses off TAM, crustal thickness varies between 39 and 45 km, and in WA, Newfoundland average ∼25 to ∼30 km, including a 5–6 km the WARS crustal thickness varies from 19 to 29 km and 20–35 km thick mafic underplate (Reid 1993, 1994). Dean et al. (2000) beneath the MBLD. SRF stacks for the GAM, VHIG, WILK and found ∼28 km thick crust on the conjugate margin offshore TAM show little evidence of Sp arrivals coming from upper-mantle Iberia, at the continental-oceanic crust transition. After account- depths. SRF stacks for WARS and MBLD suggest that there is some ing for the 5–6 km of underplating, the similarities in crustal Sp energy arriving from upper-mantle depths but arrival amplitudes thickness between the Newfoundland–Iberia margins and the do not rise above the bootstrapped uncertainties, and therefore are WARS indicate that the crust has been thinned by a compa- not interpreted. rable amount in both regions, assuming that both regions had Crustal structure in EA is similar to Precambrian crust elsewhere typical continental crust that was ∼35–40 km thick prior to around the world, where heat flow is low, suggesting that heat flux rifting. across much of EA is not elevated. Cenozoic rifts, such as the Given the similarities between the WARS and the East Africa rift system and the Basin and Range, have substantially Newfoundland–Iberia passive margin, both in terms of the rift tim- thicker crust than the WARS and may not be good analogues for in- ing and the crustal structure, the extensional processes that formed ferring heat flow in the WARS. In comparison, crustal thicknesses in them and their extensional histories may be quite similar. Conse- the WARSare similar to the Cretaceous Newfoundland–Iberia rifted quently, heat flow from the Newfoundland–Iberia basins may pro- margins and the Mesozoic-Tertiary North Sea rift, perhaps provid- vide a reasonable proxy for heat flow broadly across the WARS. ing better analogues. Heat flow in the Newfoundland–Iberia rifted Heat flow measurements for the Newfoundland and Iberia basins margins and the North Sea rift is not elevated (40 to 85 mW m−2), rangefrom40to65mWm−2 (Pollack et al. 1993;Fernandez` et al. indicating that rift-related heating of the lithosphere has dissipated. 1998; Davies 2013). These values are not elevated, which indicates By analogy, heat flow broadly across the WARS need not be ele- that the lithospheric thermal anomaly associated with rifting has vated. However, this conclusion does not rule out the possibility of dissipated. high heat flow in WA associated with localized Cenozoic extension A similar conclusion can be reached by comparing the WARSand and/or Cenozoic-recent magmatic activity. the North Sea rift, although rifting in the North Sea probably began in the Triassic, somewhat earlier than in the WARS. Nonetheless, the major phase of extension in the North Sea rift occurred in ACKNOWLEDGEMENTS the mid-Jurassic to mid-Cretaceous and crustal thickness, which is ∼20–30 km on average and 15 km at its thinnest (Beach et al. 1987; The NSF Office of Polar Programs grants 0632230, 0632239, Klemperer 1988), make it a possible analogue for the WARS. Heat 0652322, 0632335, 0632136, 0632209 and 0632185 support the flow in the North Sea is between 60 and 85 mW m−2, only slightly POLENET-ANET program. All of the seismic deployments in- higher than in the Newfoundland–Iberia passive margins. corporated in this study were made possible by the Incorporated 1646 C. Ramirez et al.

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Antarctic rift system and Marie Byrd Land hotspot, Geology, 32, 977–980. rected to the corresponding author for the paper. http://gji.oxfordjournals.org/ at The University of Montana on September 19, 2016